Fields are basically equivalent to properties on object-oriented programming, or columns in a relational database. 

Each Field has a data type which controls what type of user interface component is used to enter data into the field and in what format that data will be stored.  Custom field types, such as repeating controls or drop-down lists that control other drop-down lists, can be implemented using the IFrame Field type, by subclassing Sitecore.Web.UI.HtmlControls.Controland implementing Sitecore.Shell.Applications.ContentEditor.IContentField or by setting the Editor property of an  Item or Template. 

Some Field types, such as Link and Image, can contain XML with attributes such as the GUID and resource path.  Every Field of a Template must exist within a Section.  Section names including Workflow, Help, Publishing, Masters, Layout, Appearance, Security, Tasks, Lifetime, Statistics, Template and System are used by Sitecore and should be avoided when possible.